PRMT: The post-translational methylation of arginyl residues is catalyzed by a family of enzymes called protein arginine methyltransferases (PRMTs). Nine human PRMT isoforms are capable of forming monomethylarginine (MMA), asymmetric dimethylarginine (ADMA) and symmetric dimethylarginine (SDMA) on targeted proteins. Type I PRMTs form MMA and ADMA, and represent the majority of identified PRMTs. Type III enzymes, such as PRMT7, only produce MMA. Like the type I and III enzymes, type II PRMTs catalyze monomethylation; however, a second round of turnover results in the biologically distinct SDMA. A fourth type, Type IV, which results in the monomethylation of the internal guanidino nitrogen, has also been identified in yeast, but no mammalian homologue has been identified. (1)